This invention relates to electromagnetic flowmeters and more particularly to systems for energizing the electromagnetic coils thereof.
In a magnetic flowmeter, a magnetic field across a flowtube generates a voltage in a fluid flowing through the tube. This voltage, which is proportional to the flow rate, is sensed by electrodes and is amplified by a signal processing system to produce an output signal which is proportional to the flow rate.
The magnetic field across the flowtube is generated by electromagnetic coils excited by a driver circuit. Presently known magnetic flowmeters utilize either alternating current or pulsed direct current driver circuits. The advantages and disadvantages of each type of driver circuit are discussed in U.S. Pat. No. 3,783,687 to Mannherz et al and in U.S. Pat. No. 3,965,738 to Watanabe, for example. Although, as pointed out in these patents, a pulsed direct current drive provides numerous advantages, perturbations (spikes) caused by the rise of current in the magnetic coils and fluctations in the "steady state" current degrade the accuracy and precision of the meter. U.S. Pat. No. 4,325,261 to Freund Jr. et al, co-assigned with the present application, addresses some of these concerns.
It has been found that the voltage required to produce the current necessary to excitate the electromagnetic coils, and thus to generate the magnetic field, varies with cable length, the resistance of the electromagnetic coils, and with coil temperature. In addition, this voltage can vary with the particular wire size used in the electromagnet, with manufacturing tolerances, and with differences in eddy current losses. Such variations in voltage reflected in coil current tend to cause drift and other errors in the flowrate sensed by the flowmeter and tend to mask the actual flow signal.
As mentioned above, a DC driver circuit is desirable for use with such magnetic flowmeters, and in particular a squarewave regulated current is particularly desirable. Heretofore, however, such a squarewave driving current has required two voltage supplies, both positive and negative. In addition, such systems, in order to conserve power, have used duty cycle excitation of the coils, which has resulted in a large pulsating component in the flow signal itself. Such duty cycle excitation has been used because with an inductive circuit, such as the coil of an electromagnet, the current never reaches a steady-state value when driven by a fixed voltage drive. The prior art devices have used duty cycle excitation to provide the desired average current through the electromagnet coils. However, this has resulted in undesirable ripple in the coil current which causes the pulsating component in the flow signal referred to above.